Neuroimaging techniques permit the invivo evaluation of the human brain, allowing a better understanding of its anatomical, functional and metabolic substrate. This chapter reviews the literature which uses MRI for morphometric evaluation of the brain in studies related to anxiety disorders, with an emphasis on panic disorder (PD), generalized anxiety disorder (GAD), social anxiety disorder (SAD), and simple phobias. The neurobiology of GAD seems to involve neurochemical, neuroendocrine, neurophysiological, and genetic factors. Recent functional neuroimaging studies in patients with SAD suggest that the medial and orbito-prefrontal areas may be involved in the physiopathology of the disorder. Structural neuroimaging studies of GAD are still in an early phase. The studies reviewed in the chapter involve highly heterogeneous samples, reflecting the methodological difficulties in conducting studies of this disorder in samples with a precise diagnosis and with no comorbidities.